1. Field of the Invention
This invention relates to the measurement of angular speed and, more particularly, to a fibre-based interferometric gyrometer using the Sagnac effect, the angular speed being determined by interferometric measurement.
2. Description of the Prior Art
Interferometric gyrometers are known which use a wound optical fibre simultaneously traversed by two light waves travelling through the fibre in opposite directions. When the assembly is rotated about the axis of the wound fibre, one of the waves travels in the direction of rotation and the other in the opposite direction. The two waves emerging from the ends of the fibre are phase-shifted and this phase-shift is proportional to the speed of rotation. This phase-shift is displayed by recombining the emerging waves by interference. The level of the detected signal is related to the phase shift between the two waves and complies with a law in Cos.sup.2. In order approximately to obtain the proportionality between the level of the detected signal and the phase-shift at low speeds and to change the direction of the phase-shift at the same time as the direction of the speed, a phase-shift equal to .pi./2 is introduced between the two waves so that, at zero speed, the level of the detected signal is a mean level which increases or decreases according to the direction of rotation.
With an arrangement such as this, it is not possible to overcome the drifts inherent in the apparatus, namely: the drift of the amplifier, the drift of the output power of the laser etc. In order to reduce the lowest measurable speed and to approach the lower limiting speed capable of being detected, it is possible to introduce a modulation followed by a synchronous demodulation which enables some of the noise of the system to be eliminated.